Mineral particles, such as calcium carbonate, dolomite, calcium sulphate, kaolin, talc, titanium dioxide and aluminium hydroxide, are often used as fillers and pigments for making cellulosic products, such as paper and paper board. These inorganic materials are incorporated into the fibrous web (or wood pulp slurry) to improve the quality of the cellulosic product. Without such fillers, cellulosic products have poor texture due to discontinuities in the fibrous web. Moreover, the printing characteristics, opacity, brightness and bulk of cellulosic products all benefit by the addition of fillers.
Fillers improve printing characteristics of paper or paper board products by improving surface smoothness, and improving the opacity and brightness of a sheet of paper of a given weight. Additionally, fillers increase the bulk of the cellulosic product, which is important because paper is sold by area, not by weight. Bulky paper can be calendared or "finished" more than thin paper to produce a smoother sheet, which prints better.
However, the web strength of paper products generally declines as filler is substituted for fiber. Preferred fillers are therefore chosen to have minimal impact on fiber-to-fiber bonds while maintaining the strength of the paper web, particularly at high filler levels. There are two basic mechanisms that control retention of a filler in a paper web. These mechanisms are filtration retention and adsorption retention.
The properties which make inorganic materials valuable as fillers are known. They include low abrasion, as well as high brightness and opacifying characteristics. Low abrasion is required so that the cellulosic product can be manufactured easily with conventional machinery. The brightening and opacifying characteristics are important in producing paper or board products which incorporate whiteness, high opacity, good printability and an optimum bulk/weight ratio. The brightness and opacifying characteristics of a filler when incorporated in a sheet of paper may be quantitatively related to a property of the filler known as the "scattering coefficient" or "S." The scattering coefficient is routinely considered in papermaking and has been the subject of many technical papers.
A frequently used filler is titanium dioxide, which can be incorporated into the paper in the form of anatase or rutile. Titanium dioxide has a higher refractive index than other naturally occurring minerals or cellulose fiber and therefore increases the opacity of the paper. However, titanium dioxide is expensive and very abrasive. Thus, despite titanium dioxide's effectiveness as an opacifying filler, its use as a filler is limited and cheaper more satisfactory replacements are desired.
Calcined kaolins are another class of materials used as fillers in papermaking. These minerals are structured, i.e. formed from platelets interconnected or bonded together to form aggregates. The aggregates create a high number of internal voids or pores, which function as light-scattering centers. Unfortunately, these calcined kaolins have higher abrasion than natural or un-calcined kaolins, and are relatively expensive to produce. This limits their applicability as paper fillers. Calcined kaolins, because of their low bulk density, also disrupt the fiber-to-fiber bonds of the paper web at high filler loads, thereby significantly weakening the strength of a sheet of paper relative to other higher bulk density fillers.
Many paper products are manufactured in the neutral or alkaline pH range. These products are very amenable to the inclusion of alkaline metal carbonates and sulfates as fillers, unlike papers made in the acid pH range. One known method for preparing a precipitated alkaline earth metal carbonate is to calcine a naturally occurring metal carbonate, such as limestone or dolomite, to drive off chemically combined carbon dioxide. The alkaline earth metal oxide is slaked in water to form a suspension of the metal earth hydroxide, and then carbon dioxide is passed under controlled conditions through the suspension of the metal hydroxide.
Passaretti et al. (TAPPI Journal, Vol. 76 No. 12, 135-140, 1993) describe a range of precipitated alkaline metal earth carbonate fillers and compare them to other fillers. The scalenohedral form of precipitated calcium carbonate imparts the most opacity and bulk to a sheet of paper by virtue of its morphology, which contains internal voids that scatter light. Scalenohedral precipitated calcium carbonate fillers provide bulk to the sheet of paper and can effectively replace titanium dioxide, despite its lower pigment refractive index. However, the high internal pore volume of these fillers substantially retards the drainage of the paper web, and can result in significant slowing of the production rate of a paper machine.
The volume imparted to the cellulosic product by precipitated scalenohedral calcium carbonate, like calcined kaolins, also significantly weakens the fiber-to-fiber bonds in a paper sheet at high filler levels.
Prismatic or rhombohedral precipitated calcium carbonate particles, which contain little or no internal pore volume, can be incorporated into the sheet of paper to help the cellulosic web drain. Manufacturing these precipitated calcium carbonate fillers requires that the reaction temperature of the precipitation process be controlled at very low temperatures. This requires chilling equipment and results in expensive pigments. A major limitation of "prismatic" or rhombohedral precipitated calcium carbonate mineral fillers is that they do not enhance the bulk of a sheet of paper.